Such a method is disclosed by DE 197 13 085 A1 under the designations vacuum build-up testing and vacuum reduction testing. After opening of the tank venting valve, the vacuum prevailing in the inlet manifold ensures that the fuel-air mixture present in the tank venting system including the tank is sucked out, with the result that a vacuum builds up in the tank venting system. If the vacuum threshold is not reached within a predetermined length of time, this is already indicative of a leak in the tank venting system. In order to be able to roughly estimate the size of the leak, the attainment of a minimum pressure value in excess of the vacuum threshold is verified. If the minimum pressure value has not been attained, a medium-sized leak is inferred. If it was not even possible to attain the minimum pressure value, this is indicative of a major leak or the absence of a fuel filler cap. If the vacuum threshold was attained, the tank venting valve is closed again. In the case of a leak-tight tank venting system scarcely any pressure increase, if any, will be measurable. If a pressure increase occurs, however, which means that air or gas is getting into the system through a leak, the size of the leak is determined on the basis of the time curve for the pressure build-up, this being accomplished in DE 197 13 085 A1 by means of a physical model.
In the known method of vacuum build-up testing the tank venting valve is activated in such a way that the passage cross section of the recovery line is continuously increased up to a predefinable diagnostic value. The predefinable diagnostic value thereby serves for predefining a desired gas mass flow through the tank venting valve.